First Scratch Build- Tons of Random Questions as I encounter them!

[sanstoetter]

I'm in the process of designing/building a plane from scratch for a school project (due 12/3). The plane will be made primarily from birch plywood/foam/carbon rod, using the FT Power Pack C , Spektrum 6 channel receiver. Right now I'm guesstimating it'll weigh somewhere in the neighborhood of 800g, have a 42"x8" wings, and a wing cube loading of around 8.6. Currently I have a sport cub S trainer that's fine for neighborhood flying, but I'd like something for larger areas. The end goal is something that'll be easy enough to transition into, but have more sport/aerobatic characteristics. Something similar to the Mini Ultra Stick, but toned down a bit.


Photos:
http://imgur.com/a/GmQOR


Current Questions:
1. Will my current wings be strong enough? The wings will be made with 0.75" foam board sandwiching a 40"X 1/8" carbon rod, a 40"x 3/16" carbon rod, and maybe two 40"x 1/4" carbon strips.

2. Is there a standard stabilizer size I should try to hit? The only info that I've found so far says the vertical/horizontal stabilizers should be roughly 1/3 the wing area (~5"x7").

3. Will a Clark Y Airfoil work well for this plane (Clark Y 11.7% Smoothed w/ 8" Chord), and if so what AoA should I consider? I've been thinking somewhere between 0-2 degrees.

4. Will a symmetrical airfoil work well for the stabilizers? I've already cut templates for a joukovsky 15% with a 5" chord.

5. Do I need to add rod to the elevators/rudder/ailerons, or will a covering film provide enough support?

6. How large should I cut out the control surfaces? I don't want to overload the Emax 9g servos.

7. Will the supplied 10x4.7 props work, or will I need more speed? I have a 2700mah 40C 11.1V battery that I'll be using.

8. Do you see any unnecessary weight that can be cut (center wing support, middle firewall support,etc?)


As I get further I'm sure that Ill have more questions, but if I'm way off base at this point I'd like to know so that I can fix it before my first flight.

 

[Tench745]

I'm not exactly following how you intend to construct your wings/etc. Are you intending to use the FT style folded wing?
Also, .75" foam board? That is HUGE. Standard Dollar Tree foam board is 3/16" thick, or .1875" Even without the carbon spars 3/4" foam board would be pretty darn stiff.
1/3 wing area is a good rule of thumb for fin sizes. There is a bunch of math and diagrams that can get you "better" results, but it's largely unnecessary in these sorts of models.
Clark Y is a decent all-round airfoil. I usually go with 0 deg incidence on mine, but most of the time I'm just winging it anyway. (puns)
Symmetrical airfoils for your fins should be fine. You may have to shim some incidence into your Horizontal stabilizer depending on your incidence on your wing. The fins should be small enough I wouldn't think they'd need extra support. If in doubt, cut them and try to flex them by hand. If you don't like the flex you see, add some support.
Rudder and elevators (I think) are usually 1/3 or your corresponding fin area. I forget the rule of thumb on aileron size. I often just go by "That looks about right"
I would skip the center wing support, seems unnecessary to me, especially with the inherent strength of a plywood fuse. I'd leave the firewall support for crash resilience. Probably don't need as many wing spars as you have shown.

 

[sanstoetter]

I'm using 3/4" thick XPS foam to build my wings. I'll be using a hot knife to make a trench to glue the carbon spars, and then gluing another sheet of the XPS to bring the thickness to 1.5". After the glues dries I'll be using my hotwire to cut the airfoil. The goal is for a very strong wing as I'm a Noob flyer, and it'll probably see some serious G-forces trying to keep it in the air.

 

[sanstoetter]

Here's a new photo of it's current state, and it weighs 514g. The only items missing in that photo are the 4 x 9g servos, carbon spars, and of course the foam to fly. I really hope that it'll be flyable under 800g, and then less than 50g for landing gear.

I'm working on fine tuning my hotwire right now. I tried using what I had on hand, but 10a at 12v using the wire I had on-hand ran a bit too hot.

 

[HowlingWolven]

If you're worrying about your wing folding - that's not gonna fold with anything short of a direct impact on the top of the wing in combat. By something heavy and FAST.

 

[sanstoetter]

Well the wing won't end up being a full 1.5" thick, but somewhere around an inch.

Any thoughts on how large of an area for the ailerons? Should I make them almost full wing length like the Ultrastick Mini, or along the lines of something with flaps where they go deeper into the wing? I'm assuming I can make them a tad oversized, and limit the travel with either the horns or transmitter.

 

[sanstoetter]

Still working on my plane, but haven't had much time over the holiday break. My goal is to have the plane at 90% tonight as it has to be turned in Thursday morning. I'm thinking at this point it'll be a tad heavier than expected (~900g), and I should've gone with a bit more wingspan.

Wing (Pre-Hotwire)

Stabilizers

Start of foam fuselage skin

 

[paulshort]

Looking good.

 

[ProfessorFate]

Your tail stab incidence? Will it be level with thr line of wing?

Really nice looking build with great materials, just, before you get much further, you might want to adjust the end of the fuse so that it parallels the incidence of the wing or is really quite opposite of what it is set at now.

You are planning, like most do, to lay the horz stab piece flat on the carbon tail, right?

The incidence you have in the tail now is downward, so if anything, simply cut away the epoxy where you attached those CF tail rods and line them up right so they are level with the wing seat.

The way it is now, when it flies, you would have to put in a lot of up-trim in your elevator just to get it to fly level.



This picture is from the company that makes the Savage Bobber, I'm making my own wood version that's a cross between this and the FT Bushwacker.
The picture has the planes tail a little low and sloping down, but, you can see if you move the pic around in your viewer and compare the wing to the tail elevator stab, their incidence matches and also you can see the tail shape too see the top rods go straight and level from wing trailing edge to the tail stabs, and, at the bottom, these fuse structural pieces slope upward.


Looking again at your pictures, you might accept it at a minimum just to loosen and re-glue the 4 points only, but you might not like the way the tail is bowed and might want to cut the other sections loose and re-glue them too

If you do this it will fly better and look better too, you could just look and see pics of other planes as an example of this at a side view of the fuse, how the tail parallels the wing seat on a Cessna and how the bottom of the fuse slopes up

Anyway the top of your fuse slopes down

I am aware tho, some pictures of airplanes obfuscate this point, like a Hawker Hurricane has that taller portion sloping down from the elevation of the top of the canopy, and so I get where you might have thought to make the tail that shape, but still on these look at the side view and compare incidence of the wing ( or the thrust line of the airfoil of the wing ) and the elevator stab.

Well I really hope this helps

I noticed on the FT Bushwacker, they use a fat flat bottom foil 8" chord like you are planning, should work.

I took my tail apart on my Savage Bobber and re-made it 2 times

Looking good, such nice neat build techniques and materials.
I like it.
Leonard


****Oh sorry, don't see any finished pics but, your plane may already done and turned in, today is 14 Dec 2015, in the case it's not done yet, or if it takes a lot of up trim just to fly level and you don't want to rebuild a lot of it, you could just adjust the horz stab level with the set of the wing.

 

[sanstoetter]

Well I haven't updated this thread in a while, but I did finish it as far as the class was concerned. I haven't had a chance to fly it yet, but hopefully this weekend I'll give it a shot.

I ended up building the vertical stabilizer long so it fit between the carbon rods, and glued the horizontal stabilizers to the vertical.

I had to rush to get it completed enough to turn in, and made a few revisions after I got it back.

Here's a photo of the fuselage w/out wings in the process of making a foam body before covering it with shrink film.

I haven't taken a final shot yet as the body I made is super rough, and I plan to redo it after a few flights.

 

[sanstoetter]

Well I took it on its maiden flight, and need to make a few changes before flying it again. The CG was a bit too far back, and I need to redo my pushrods with a larger gauge wire.

Here's the maiden flight... FYI I'm a noob flyer, and the wife decided to only film a few seconds of the flight.
https://www.youtube.com/watch?v=h_Ojr8cXLgk